The invention relates to a method for the heat treatment of a work piece produced from a Ti alloy for obtaining a fine-grained structure by annealing the work piece above its β transus temperature (β annealing), whereby the work piece is heated in a furnace to a temperature level above its β transus temperature and the achieving of the temperature level determines the beginning of a holding time predefined as to its duration, and the work piece is held for the duration of the holding time at the temperature level before it is subjected to a cooling-off process.
Work pieces that consist of a titanium alloy are subjected to various heat treatments as a function of their chemistry and their intended use in order to impart to or adjust certain properties of the work piece. To this end, work pieces of titanium alloys are occasionally subjected to an annealing method. Depending on the alloy type and the particular desired property to be achieved, the main intended use of such annealing methods resides in an increase of the strength, the adjusting of a sufficient ductility as well as in a thermal stability and/or to increase the resistance to creeping. One of these heat treatment methods is the so-called β annealing. In this method the work piece is annealed to just above its β conversion temperature (β transus temperature) and subsequently subjected to a defined cooling-off process. The cooling-off process can be cooling in air, in an inert gas to room temperature or can also be a quenching. The hexagonal α phase contained in the Ti alloy is converted into a spatially centered β phase above the β transus temperature. The quenching process following the β annealing is typically designed to suppress the formation of α phase as much as possible during the cooling off or to separate it in a defined manner.
In the case of work pieces of Ti alloys, they can be structural components, for example, for being used in airplane construction. Such structural components typically have a not-inconsiderable thickness. During the β annealing of such a work piece particular care is required in order to achieve the desired properties. To this end standards have been developed according to which such Ti structural components must be β-annealed. The standardizing of the β annealing process is intended to ensure that during an industrial usage the work pieces have the most uniform grain structure possible. A problem in β annealing is that keeping the work piece above its β transus temperature for too long result in an undesired grain coarsening. According to the standards in force, such as AMS-H-81200B or DIN 65084, it is required that the work piece be heated to a temperature 30° C. above the β transus temperature of the Ti alloy. The temperature level to which the work piece is to be heated, which lies above the β transus temperature, has a sufficient temperature difference from the β transus temperature, which level is also ensured taking into consideration the system-conditioned temperature tolerances (β transus temperature, furnace temperature), so that the work piece is heated as a whole upon achieving the temperature level above the β transus temperature. For the adjusted furnace temperature, generally a tolerance range of ±14° C. is given. A β annealing is carried out in accordance with these settings by heating the work piece in a furnace. When the work piece temperature exceeds the lower tolerance limit of the predefined temperature level (Tβ+30° C.−14° C.) determines the start point of the holding time. The holding time itself is preset, for example, at 30 minutes. Consequently, the work piece is kept in the furnace for the duration of the holding time at a temperature level above Tβ+30° C.−14° C. and is subsequently subjected to a cooling-off process. Such a method is known in principle from GB 1,141,409. This document describes a method for the refining of the grain of the microstructure of an α or β titanium alloy. The work piece is heated to a temperature above the β transus temperature in order to obtain a substantially complete conversion into the β phase. The work piece is held at this temperature until it has been sufficiently ensured that a complete conversion into the β phase has taken place. A holding time of one hour is indicated as an example. The work piece is subsequently quenched to a temperature sufficiently far below the β transus temperature to bring a significant part of the β phase into an α phase or an α-equivalent phase. In a following step the shaped part is plastically deformed. The annealing referred to in this document is an intermediate step in the production of the material in the “annealed state” with a grain structure of globular α phase that is adjusted after the β annealing and after a further D formation. No β annealing is described in this document that represents a final heat treatment with which the grain size of the β structure is refined, as was initially mentioned.
It turned out that in spite of the normative settings for the β annealing of work pieces consisting of a titanium alloy, they were not able to be produced with the necessary process safety and that they therefore can differ from each other as regards their structure and consequently their properties in spite of the same method parameters. However, this is not desired.
Starting from this discussed state of the art, the invention therefore has the problem of designing an initially cited method in such a manner that a β annealing of work pieces consisting of a titanium alloy is possible with a higher degree of process safety.